Optical filter for spectroscopic measurement and method of producing the optical filter

ABSTRACT

An optical filter used in applications involving spectroscopic measurements is fabricated by depositing layers of optical coatings onto a substrate. The layers are deposited so as to have a substantially constant thickness in a first direction along the surface of the substrate, and a gradually increasing thickness along a direction perpendicular to the first direction. The structure of the optical filter allows for large scale production of the filter so that costs in producing the filter are greatly reduced. The filter may be used in a variety of applications including, but not limited to chemical analysis, blood glucose monitoring, and the like.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical filters which are used inapplications where spectroscopic measurements are used to determine theproperties of substances such as chemicals and other substances.

2. Description of the Related Art

Optical filters are well known in applications involving spectroscopicmeasurement. Spectroscopic measurement is used to determine theproperties and chemical composition of various substances in a samplebased upon the optical characteristics of the sample. In a typicalspectroscopic measurement, light (in the visible and non-visible range)is used to illuminate the sample over multiple frequency spectra. Morethan one optical frequency (wavelength) is used to more preciselydetermine the optical characteristics of the sample and also to subtractout interference. In some applications, the light reflected from thesample is detected, while in other applications light transmittedthrough the sample is detected to determine the optical characteristicsof the sample. In addition, a combination of the transmission throughthe sample and the reflections from the filter may be employed.

The detected light is usually quantified to provide an indication of the"frequency response" of the sample at each of the frequency spectra. Asis well known in the art, each substance has definable opticalproperties determined by the frequencies at which the substance reflectsand absorbs light. Thus, the optical characteristics of a givensubstance may be quantified (e.g., plotted as intensity of reflected ortransmitted light versus frequency) to provide an indication of theoptical characteristics of that substance. Since different substancestypically have distinct optical characteristics, quantified measurementsof the optical properties of a sample containing several substances canserve as the basis for distinguishing among or making other measurementsrelating to the several substances within a sample. Precise measurementsof the reflected or transmitted light can be used to determine theprecise concentration of the various substances within a sample.

Some present spectroscopic measurement systems use multiple lightemitting diodes (LEDs) or laser sources to provide light at the desiredwavelengths. However, very expensive, high precision wavelength lightsources must be employed in order to manufacture such a system with thenecessary wavelength accuracy for each of the sources.

One alternative method of generating light at multiple frequenciesinvolves rotating an optical filter between the sample to be measuredand a broadband light source. Current optical spectroscopic devices, asidentified by the inventor for use in the present invention, oftenrequire expensive custom-made filters which are used to generate apattern of optical signals to be transmitted. One such filter, commonlyknown as a dichroic filter, comprises a rotating optically coated diskwhich includes regions of varying optical thickness. As the wheel spins,light from the broadband light source passes through different portionsof the wheel so that light of various frequencies are passed by thefilter to illuminate the sample. That is, the regions on the dichroicfilter are formed in a pattern so that rotation of the optical diskresults in the transmission of selected optical bands. In many previousapplications involving precise spectroscopic measurement, opticalfilters have been designed with very high tolerances. Furthermore, themethods for manufacturing such filters have often precluded thepossibility of manufacturing the filters by mass production. Thus, evenoptical filters of this kind may be prohibitively expensive tofabricate.

SUMMARY OF THE INVENTION

The present invention provides a rotating dichroic filter forspectroscopic measurement wherein the cost of the filter isapproximately 100 times less than conventional rotating dichroicfilters. This is accomplished by first relaxing the specifications ofthe filter and compensating for the relaxation of filter specificationsthrough more intensive signal processing steps. In addition, the filteris constructed in a manner which allows for easier production. Thefilter constructed in accordance with the present invention allows from10 to 100 times as much light to pass while maintaining the necessaryprecision through signal processing.

One aspect of the present invention involves a method of manufacturingan optical filter. The method involves a number of steps. An opticalsubstrate is provided having a top surface and a bottom surface, andlayers of optical coating are deposited on the top surface such that thelayers vary in thickness across the top of the substrate in a firstdirection. The thickness of the layers is substantially constant in asecond direction substantially perpendicular to the first direction. Inone embodiment, the method further involves creating a mounting hole inthe center of the substrate. In addition, an opaque strip along at leasta portion of the substrate is deposited in one embodiment.

Another aspect of the present invention involves an optical filter. Theoptical filter has a substrate having a top surface and a bottomsurface. The filter also has a plurality of optical coatings depositedon the top surface of the substrate such that the coatings vary inthickness in a first direction across the top surface. The coatings aresubstantially constant in thickness across the top surface in a seconddirection substantially perpendicular to the first direction.

Another aspect of the present invention comprises an optical filterhaving a generally circular substrate. Layers of optical coatingsdeposited on the substrate provide a non-imaging interferometer whereinapproximately one-half of light incident upon the coatings passesthrough the coatings over the entire surface of the substrate.

Yet another aspect of the present invention involves an optical filter.A substrate having a top surface and a bottom surface has a plurality oflayers of optical coatings varying in thickness in a first directionacross the substrate. The layers provide optical transmissioncharacteristics for the optical filter to provide an optical filterwhich transmits more than one wavelength through the filter at alllocations across the surface of the filter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an exemplary dichroic filter as constructed byconventional methods.

FIG. 2 depicts schematically the general method used in accordance withthe present invention to manufacture a rotational optical filter.

FIG. 3 depicts the dichroic filter of the present invention depicted inFIG. 2 in a blood glucose monitoring application.

FIGS. 4A-4C depict in graph form the optical transmissioncharacteristics for an exemplary dichroic filter over different degreesof rotation in accordance with the present invention.

FIG. 4D illustrates a matrix used to specify the optical characteristicsof an exemplary dichroic filter in accordance with the presentinvention.

FIG. 5 depicts in graph form the optical transmission characteristics ofan exemplary conventional dichroic filter over different degrees ofrotation in accordance with the present invention.

FIG. 6 depicts a general flow chart of the signal processing operationswhich are used to compensate for the lower optical tolerances of thefilter of the present invention.

FIG. 7 illustrates a flow chart which sets forth the general steps ofobtaining the optical characteristics matrix of FIG. 4D.

FIG. 8 represents a functional block diagram of the general steps ofusing the filter of the present invention in conjunction with signalprocessing to accommodate imprecision.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an exemplary dichroic filter fabricated according toconventional methods. Previous methods employed to fabricate suchoptical filters typically involved laying out a circular substrate andthen selectively increasing the coating thicknesses on the surface ofthe circular substrate as the substrate is rotated with uniform speed.

Such a filter 150 is depicted in FIG. 1 as having coating layers 152,154, 156, etc., of increasing thicknesses to form a spiral configurationas the filter 150 is rotated. Of course, it should be understood thatthe coating thicknesses depicted in FIG. 1 are exaggerated for ease ofillustration. This method of optical coating is carried aroundsubstantially the entire circumference of the circular substrate so thatas the coated substrate revolves, the thickness of the optical coatinggrows throughout the entire revolution and then suddenly drops back fromthe thickest coating to the thinnest coating at the end of onerevolution.

It has been found, however, that such methods of optical coating requirehigh precision and are extremely costly. Furthermore, manufacturingthese filters is typically carried out one-by-one, since productionmethods do not allow for laying out several disks on a single sheet formass production purposes.

In addition, conventional filters of the type depicted in FIG. 1generally have many layers (e.g., 100 or more layers is common). Thenumber of layers in conventional filters are provided to provide veryprecise pass bands (for a bandpass filter). FIG. 5 depicts an exemplarytransmission characteristic for a conventional rotational dichroicfilter versus degrees of rotation for a selected wavelength. Asillustrated in FIG. 5, the pass band of the filter is very precise forthe selected wavelength, generally without side-lobes, and also providesessentially zero transmission outside the pass band. A very high numberof layers is required to obtain a filter with this near ideal precision.It should be understood, that this very narrow passband is in differentrotational positions for different wavelengths. In other words, aconventional dichroic filter can be characterized as a monochrometerwhich passes a different wavelength at different rotational positions.

Creating each layer is expensive due to the continuous rotationalvariation from thin to thicker. Thus, when many layers are created(e.g., 100 or more for good precision), such conventional filters arevery costly.

In accordance with the present invention, a dichroic filter is disclosedwhich differs significantly from conventional dichroic filters. FIG. 2depicts a filter 120 along with the steps followed in the method ofproducing a filter in accordance with the teachings of the presentinvention.

The dichroic filter according to the present invention is made in anovel manner in which the multiple optical coatings are created on asubstrate to form a wedge-like substrate. For a rotational filter, thesubstrate is then cut to form a rotational disk filter.

In addition, according to one aspect of the present invention, thedichroic filter has fewer layers than conventional filters. Thisprovides for less precision in the transmission characteristic of thefilter. FIGS. 4A-4C depict the optical transmission characteristics forselected wavelengths of an exemplary rotational filter made inaccordance with the present invention having only 17 optical coatinglayers. As illustrated in FIGS. 4A-4C, the transmission characteristicis not as precise as the transmission characteristic of the filterrepresented in FIG. 5. As depicted in FIGS. 4A-4C, the dichroic filterof the present invention has a several pass-bands for each wavelengthdepicted. In addition, outside the pass-bands, the transmission does notfall completely to zero, as with the conventional precision filters. Thereduced precision in the passbands is due to the reduced number oflayers in the filter. It should be understood, that the reducedprecision explained above is not limited to rotational dichroic filters,but could also be advantageous with dichroic filters that are vibrated(e.g., through oscillation or the like), and for any other opticalfilter which conventionally involves high precision in the pass-bands.The decreased precision of the filter of the present invention isaccommodated with signal processing as further explained below to obtainthe required precision. In this manner, the cost of the filter can bereduced.

When both aspects of the filter in accordance with the present inventionare used (layering process and reduced number of layers), the resultingfilter is much less expensive to construct than conventional dichroicfilters. However, it should be noted that using either aspect ofreducing cost is advantageous in itself. For instance, a conventionalrotational filter could be fabricated with far fewer layers, but usingconventional layering techniques such that the filter increases inthickness through the entire revolution of the filter. Alternatively,the method of fabrication disclosed herein could be used to form arotational filter with conventional precision (e.g., many layers) atreduced manufacturing costs due to the improved manufacturing method.

In the method which reduces the cost of layering the optical filter, aflat substrate 110 (FIG. 2) is coated with optical coatings ofincreasing thickness to form a wedge-shaped coated layer 111. It shouldbe noted that for purposes of clearly illustrating the presentinvention, the thickness of the optical coating 111 has beenexaggerated, and in practical applications the thickness of the opticallayer 111 varies from roughly 1.66 micrometers to about 3.33micrometers, with an average thickness of about 2.35 micrometers. Itshould also be understood that these thicknesses are approximate and mayvary depending upon the index of refraction of the layer materials.Therefore, in accordance with one aspect of the present invention, theoptical coatings which define the filter are applied across a substraterather than continually applying coatings circumferentially, thus,significantly reducing the cost of the filter. The filter at this pointprovides a dichroic filter which could be used in oscillating filtertype applications.

For a rotational filter, once the optical layers 111 have been appliedto the substrate 111, a cylindrical portion 112 is cut from thewedge-shaped slab formed by the optical layer 111 together with thesubstrate 110. A cylindrical aperture is then formed in the center ofthe cylindrical portion 112 to form a mounting hole. In certainapplications, it is desirable to form an optically opaque strip such asa brass strip 122 over a portion of the optical filter disk 120. Thebrass strip provides a zero-transmission reference portion of the disc120 which may be helpful for noise cancellation in certain signalprocessing applications.

The above description provides ease of illustration for understandingone aspect of the present invention. However, it should be understoodthat the method may, in practice, involve first cutting the substrateinto a disk. Thereafter, the optical coatings are applied onto the diskas though the disk were still square so that the excess falls onto theplatform (not shown) supporting the disk within the vacuum tank. In thismanner the wedge is formed on the surface of the disk 120 as shown inFIG. 10.

It will be understood that the disk 120 does not continually increase inthickness through the entire circumference of the wheel, but increasesin thickness and then decreases in thickness. However, both halves ofthe circumference can be utilized as further described below.

In addition to the reduced manufacturing cost of the filter describedabove, in accordance with a further aspect of the present invention, aminimal number of optical coating layers are deposited. In one preferredembodiment, only 17 layers are necessary to obtain the desiredresolution.

Although reducing the number of layers results in less precise filters,such imperfections can be accommodated in digital signal processingsteps. For example, as explained above, conventional dichroic filterstypically pass a single frequency band at a time (FIG. 5), while thefilter of the preferred embodiment may allow for multiple bands to pass,since this is accounted for, and can be compensated through signalprocessing.

It should be noted here that the resolution typically necessary forapplications involving more expensive interferometers or monochrometersis typically not necessary for analyzing liquids. However, additionallayers can be added at greater spacing intervals in order to increaseresolution of the filter.

COMPENSATING DIGITAL SIGNAL PROCESSING

As briefly set forth above, the imprecision of a filter made inaccordance with the present invention having a minimal number of opticalcoatings can be accommodated through signal processing.

FIG. 6 is a data flow diagram which details the method used tocompensate for the imprecision of the filter made in accordance with thepresent invention. It should be understood, however, that prior torun-time, initialization is performed.

PRE-RUN-TIME INITIALIZATION

The initialization is performed at the factory or other time prior touse. In general, a filter characteristics matrix is constructed, asdescribed in greater detail below with reference to FIG. 7. The filtercharacteristics matrix represents the transmission characteristics ofthe dichroic filter 120 at different portions of the filter 120 and forvarious wavelengths of light. The filter characteristics matrix is usedin order to extract portions of the electrical signal generated by adetector which are due simply to the optical attenuation caused by thefilter 120. In other words, by knowing the filter characteristics, theimpression of the filter can be accounted for.

The filter characteristic matrix is a two-dimensional matrix. The filtercharacteristic matrix includes one column for each wavelength of lightwhich is characterized and one row for each position (rotational in thepresent invention) of the filter 120, at which characterization (of thefilter characteristic) is performed. Thus, in one embodiment, the filtercharacteristic matrix includes 16 columns and 256 rows when 16wavelengths are characterized and 256 positions of the filter 120 aredefined. It should be understood here that it is not necessary that 16different wavelengths be used; the use of additional wavelengths isparticularly advantageous for increasing the signal-to-noise ratio.Since about half of the incident light is transmitted through the filterat each position of the filter, the same wavelength is detected multipletimes (although in a unique combination with other wavelengths eachtime) so that the overall signal intensity is from 10 to 100 times theintensity of any single wavelength and much higher than the noise floor.This is commonly referred to as Felgate's advantage. In this manner thespectral response of the entire filter 120 over the expected measuredwavelengths is completely characterized. The method employed toconstruct the filter characteristics matrix is described in detail belowwith reference to FIG. 7.

DERIVATION OF THE FILTER CHARACTERISTIC MATRIX

FIGS. 4A-4D, together with FIG. 7, illustrate in greater detail, themethod employed to obtain the filter characteristic matrix. Thederivation routine is illustrated in FIG. 7 and starts with a beginblock 800.

The activity blocks 830-845, together with FIGS. 4A-4D, illustrate themethod used in accordance with the present invention to construct thefilter characteristics matrix. The filter 120 reflects and transmitsoptical radiation in different proportions for different wavelengths atdifferent places on the filter disk 120. This is clearly illustrated inFIG. 4A-4C, wherein FIG. 4A represents the optical transmission of lightat a wavelength of 850 nanometers plotted versus each of a possible 256disk rotational positions (for one embodiment). As shown in FIG. 4A,when the disk 120 is in the initial starting position (i.e., φ=0 where 0represents the rotational position of the filter 120) , the transmissionof light at 850 nanometers is approximately 10% through the filter 120,while when the disk 120 is rotated so that 0=32, the opticaltransmission of light at 850 nanometers through the filter 120 isapproximately 25%. Again, between the disk rotational positions of φ=128to φ=160, the transmission of light at 850 nanometers wavelength throughthe filter 120 is approximately 75%. Thus, the optical transmission forλ=850 nanometers is entirely characterized over 256 rotational positionsof the disk filter 120, as depicted in FIG. 4A.

FIG. 4B depicts the optical transmission characteristics of light at1,150 nanometers over the same 256 rotational positions of the disk 120.Similarly, FIG. 4C depicts a plot of the optical transmission of lightat 1,350 nanometers through the disk filter 120 at each of the 256rotational positions of the disk 120. In one actual embodiment of theinvention, the optical transmission characteristics of the filter 120are described for 256 rotational positions at each of 16 wavelengthsbetween 850 nanometers and 1,400 nanometers.

Thus, from these measurements, a filter characteristic matrix may beconstructed, as shown in FIG. 4D. The filter characteristic matrixdesignated in FIG. 4D as F(φ,λ) includes 256 rows and 16 columns. Eachcolumn of the filter characteristic matrix comprises the spectraltransmission characteristics of the disk 120 at each of the 256rotational positions of the disk 120 for the selected wavelength forthat column.

In order to construct the filter characteristic matrix depicted in FIG.4D, the filter 120 is illuminated at a first rotational position overeach of the 16 wavelengths to obtain spectral transmission coefficientsfor each of the 16 wavelengths, as indicated within an activity block830. Once the spectral transmission coefficients have been determinedfor the first rotational position as indicated within the activity block830, the filter is illuminated at a second rotational position (i.e.,φ=1) over the 16 selected wavelengths to obtain spectral transmissioncoefficients for the second rotational position, as represented in anactivity block 835. This method is carried on for each of the possiblerotational positions of the disk 120 until, as indicated within anactivity block 840, the filter is illuminated at the "mth," or last,rotational position (i.e., position 256) of the disk filter 120 over the16 selected wavelengths to obtain the spectral transmission coefficientsfor the last rotational position In one preferred embodiment, where astepper motor is used, the rotational positions will be precise fromrevolution to revolution of the disk 120. Of course, a computer discmotor with salient poles and run at a constant speed could be usedprovided that phase dithers are minimized to less than one part in 256.

Once spectral transmission coefficients have been determined for all 16wavelengths of all 256 rotational positions of the disk 120, the filtercharacteristics matrix is constructed, as indicated within an activityblock 845. The matrix defined by column and row where columns representcoeffluents and row represents the wavelength by putting coefficients.Once the filter characteristics matrix is constructed, the system hasthe necessary constraints for processing.

It should be understood that derivation of a filter characteristicmatrix has been described for purposes of the rotational filter 120.However, an oscillating filter, or any filter with defined positions onthe filter such as Fabry-Perot type filters and even fixed filters suchas those used in CCD applications can also be characterized inaccordance with the discussion above.

RUN-TIME PROCESSING

Discussion of the overall processing in accordance with the presentinvention in order to account for impression of the filter through theuse of the filter characterization matrix is made with reference toFIGS. 3, 7 and 8.

FIG. 3 illustrates the use of the filter 120 in a system for monitoringblood constituents. FIG. 6 illustrates a general flow diagram for thesteps of accounting for the imprecision in the filter to obtain thecharacteristics of a medium under test. FIG. 8 illustrates a generalfunctional diagram of the process of accounting for filter imprecisionthrough signal processing. As depicted in FIG. 6, the start ofprocessing is represented in a begin block 300. First, housekeeping andself-testing procedures are performed, as represented in an activityblock 305. Briefly, housekeeping and self testing involves bootoperations and conventional initialization a self testing. For example,the system first determines if there is a sufficient signal intensity totake an accurate reading. After housekeeping and self testing iscompleted, the light source 110 (FIGS. 3 and 8) is activated to transmitlight 115 through the filter 120, as represented in an activity block310. Initially, the light source 110 is activated while no test medium131 is interposed between the filter 120 and the detector 140. Thus, thelight which is detected by a detector 140 (FIG. 3) represents a baselinelight intensity (I₀) which can be used as a test to insure that a bulbwhich is too dim or too bright is not inserted as a replacement bulb forexample. In one embodiment, a lens 117 (FIG. 8) can be provided betweenthe light source and the filter 120 to provide focused light 115 on thefilter 120.

Once the initial baseline light intensity constant has been determined,the medium 131 under test is inserted as indicated in an activity block312.

As indicated within an activity block 315, the light which is incidentupon the detector 140 is converted to an electrical signal and thissignal is amplified in a pre-amp (not shown), filtered with the bandpass filter (not shown), and sampled by an analog-to-digital converter142. Since the filter 120 is rotating (at approximately 78.125revolutions per second in one actual embodiment, although otherrotational rates could be advantageous as called for by the particularapplication), samples of the electrical signal output by the detector140 are indicative of the light intensity detected at various rotationalpositions of the filter 120. In one advantageous embodiment, onecomplete rotation (i.e., 360°) of the filter 120 corresponds to 512digital samples. That is, 512 samples are taken within the periodcorresponding to one revolution of the filter 120. Thus, for example, ifthe filter 120 rotates at 78.125 revolutions per second, then 512samples will be taken within approximately 1/78th of a second, so thatthe sampling rate of the analog-to-digital converter 142 will beapproximately 40,000 samples per second.

As described, the filter 120 constructed in accordance with the presentinvention Includes redundant regions within an entire revolution.Specifically, the filter 120 is symmetrically layered so that the firsthalf-revolution of the filter provides a mirror of the signal of thesecond half-revolution of the filter 120. That is to say, as depicted inFIG. 2, the filter is formed in a wedge shape so that the thickness inone direction is constant and the thickness in the perpendiculardirection increases linearly. Thus, the second half-revolution of thefilter 120 is redundant. For this reason, digital samples taken forone-half of the revolution of the filter 120 could be discarded so thatin each rotation of the filter 120 there are 256 samples used forpurposes of digital signal processing rather than 512 samples in theembodiment described above. Alternatively, all 512 samples can be usedfor processing by averaging corresponding values. In yet an alternativeembodiment, the redundant half of the filter may be used for filter andsource calibration. Each of the 256 samples (if only half are used)represents a different portion of the filter 120 having differentoptical transmission characteristics.

Advantageously, the filter 120 is specially designed to include anopaque strip (i.e., the brass strip 122). The digital signal processor145 detects when the opaque strip 122 of the filter 120 is interposedbetween the light 115 and the detector 140 by monitoring the intensityoutput from the detector 140. This intensity is effectively zero whenthe light is blocked by the opaque strip 122. Since the opaque strip 122blocks substantially all of the optical radiation transmitted from thesource 110, any signal output from the optical detector 140 when thelight is blocked (e.g., from ambient light, thermal effects, etc.) ,will be interpreted as electrical noise which is not due to either thespectral absorption characteristics of the medium under test 131 or thespectral transmission characteristics of the filter 120. Thus, thedigital signal processor 145 interprets the signal present at the outputof the optical detector 140 when the brass strip 122 is interposedbetween the light source 110 and the optical detector 140 as stochasticnoise which is subsequently subtracted from all signals output from theoptical detector 140. In one embodiment, this is simply accomplished bysubtracting the digital value corresponding to the detected noise levelfrom each of the digital values corresponding to the detected signalsamples obtained within the activity block 315. Alternatively, a shuttermechanism could be interposed within the light path, or the lamp 110could be turned off momentarily to provide the same effect. In thismanner, the electrical noise inherent within the system is removed sothat those electrical signals due to the optical transmissioncharacteristics of the filter 120 (and the test medium 130) areconsidered in the further processing steps.

Once the stochastic noise inherent within the system has been extracted,control passes from the activity block 315 to an activity block 323.Within the activity block 323 the signal is divided by I₀ to normalizethe signal. The normalized signal is subsequently processed within anactivity block 325 to construct a signal intensity matrix, or vector,from the sample values obtained within the activity block 315 (takinginto consideration the subtraction of the electrical noise, and thesignal normalization performed in the activity block 323) FIG. 8illustrates a signal intensity matrix I.sub.φm.

The signal intensity matrix 1000 (FIG. 8) is a one column matrix(sometimes referred to as a vector) including 256 signal intensityvalues (e.g., one value for each sampled rotational position of thefilter 120 in the present embodiment). Thus, the signal intensity vector1000 is obtained by direct measurement of the optical signal whichpasses through both the filter 120 and the test medium 131 and isdetected by the optical detector 140. Of course, the values used to formthe signal intensity vector 1000 are taken from the amplitude of thesignals output from the detector 140 after subtraction of the noise fromeach sample. Designating each rotational position of a filter 120 whichis sampled by the analog-to-digital converter 170 by the symbol φ, thenφ₁ will correspond to the first rotational position of the filter 120,φ₂ will correspond to the second rotational position of the filter 120,to φ₂₅₆, which corresponds to the last rotational position of the filter120 before φ₁ is taken again. Using this notation, I.sub.φ1 correspondsto the intensity of light detected by the optical detector 140 when thefilter 120 is in the first rotational position φ₁, I.sub.φ2 correspondsto the intensity of light detected by the detector 140 when the filter120 is in the second rotational position φ₂, etc. Thus, the signalintensity matrix comprises a single column matrix having 256 digitalvalues from I.sub.φ1 to I.sub.φ256, which correspond to the opticalintensities detected at each of the rotational positions of the filter120. In one embodiment, the intensity values for several revolutions areaveraged to form the signal intensity matrix.

Once the signal intensity vector has been obtained activity block 325(FIG. 6), hereinafter designated as I(φ) , and the filtercharacteristics matrix, hereinafter designated as F(φ,λ) , has beenobtained as explained above and represented as a data input in a block333, the signal intensity matrix together with the filtercharacteristics matrix may be used to obtain a matrix indicative only ofthe optical absorption characteristics of the test medium 131, asrepresented in activity blocks 330, 331. That is, since the overalloptical absorption is known as measured within the signal intensitymatrix, I(φ), and the optical transmission characteristics of the filter120 are known as represented by the filter characteristics matrix,F(φ,λ) the optical absorption of the detected light due to thecharacteristics of the test medium 131 may be determined by removing theoptical transmission characteristics due to the filter from the overallintensity vector I(φ) combined. This is accomplished by first taking theinverse transform of the filter matrix, as represented in the activityblock 331, and subsequently multiplying the signal intensity vector I(φ)by the inverse filter matrix, as represented in the activity block 330.

If the transmission through the test medium 131 is designated as T(λ)wherein the transmission of light through the test medium 131 is definedas a function of the wavelength, and the transmission of light through aselected rotational position (e.g., when f=0, corresponding to 0°) of afilter 120 is maintained as a function of wavelength and is designatedby the function F(φ,λ), the combination, or convolution, of the opticalabsorption due to the test medium 131 and the filter 120 is designatedover the same wavelengths by the function I(φ). To obtain T(λ) from theintensity vector I(φ) and the filter transmission matrix F(φ,λ), theintensity vector I(φ) and the inverse F⁻¹ (φλ) are multiplied.

The functions I(φ) and F(φ,λ) may be represented by the signal intensityand filter characteristic matrices, respectively. Thus, since

    I(φ)=F(φ,λ)×T(λ)               (1)

and I(φ) represents a one-column matrix (vector) containing an intensityvalue for each rotational position value φ, while F(φ,λ) represents atwo dimensional matrix containing a filter transmission coefficientvalue for each value of φ and each value of λ (FIG. 4D), then thefunction T(λ), representative of optical transmission through the testmedium 131, may be represented as a one column matrix having values foreach of the various wavelength values, λ.

In accordance with one embodiment of the present invention, 16wavelengths are selected over the range of 850 nanometers to 1,400nanometers for purposes of characterizing the spectral characteristicsof the test medium 131 as well as the filter 120.

The matrix form of equation (1) above is shown below: ##EQU1## As shownin Equation (2), the signal intensity matrix I(φ) is equal to theproduct of the two dimensional filter characteristic matrix, F(φ,λ), andthe single column test medium matrix T(λ). In this equation, two of thematrices are given (i.e., I(φ) and F(φ,λ)). Thus, the third matrix,T(λ), which represents the optical transmission characteristics of thetest medium 131 for the 16 selected wavelengths between 850 nanometersand 1,400 nanometers, may be obtained by simply multiplying the inverseof the filter characteristic matrix, designated as F⁻¹ (φ,λ), by thesignal intensity matrix, I(φ), using conventional matrix inversion andmultiplication techniques, as shown below. ##EQU2## Thus, as indicatedin an activity block 331, the inverse transform is taken of the filtercharacteristic matrix, F⁻¹ (φ,λ), and then this inverse matrix ismultiplied by the signal intensity matrix, I(φ), within the activityblock 330 to obtain the frequency response of the test medium 131 asexpressed by the test medium characteristic matrix, or transmissionvector T(φ).

FIG. 8 illustrates this operation in pictorial form. As shown in FIG. 8,the light source 110 emits light which passes through the lens 117 andthe filter 120 to provide filtered optical radiation 125. The opticalradiation 125 passes through the medium under test 131 to provide anoptical signal used to generate the signal intensity matrix 1000.

The signal intensity matrix 1000 is multiplied by the inverse of thefilter characteristic matrix 1010 as indicated within a block 1005. Asshown in FIG. 9, the filter characteristic matrix 1010 is derived froman analysis of the filter 120, as described above. The inverse transformof the filter characteristic matrix 1010 is multiplied by the signalintensity vector 1000 to obtain the optical frequency response matrix,or transmission vector, 1015.

Further processing depends on the desired analysis of the test medium.

APPLICATIONS OF THE FILTER

The optical filter of the present invention has uses in variousapplications. In practice, the optical filter could be used with anyapplications where optical radiation is distinguished into multiplespectra. For example, particular benefits of the invention may beexhibited in on-line, in-stream chemical process analyzers, orindustrial applications where fast, small, low-cost instruments areneeded. It should be noted that the circular scanning technique usedwith a rotating dichroic filter provides a significant mechanicaladvantage which provides for an increased scanning speed over linearsinusoidal oscillation or a sawtooth scan. Thus, the polychrometerfilter wheel 120 has many applications in the process control industry,where rapid, real-time spectra are desired for on-line process control.Specific applications include petroleum distillation processes wheredifferent proportions of hydrocarbons are to be determined, drug andalcohol in vivo blood testing, etc.

APPLICATIONS INVOLVING BLOOD GLUCOSE MONITORING

One particularly advantageous application of the filter of the presentinvention involves monitoring blood glucose levels within a patient,such as a diabetic, without requiring the extraction of blood. Thisapplication is described briefly below.

FIG. 3 schematically depicts the filter 120 in operation as an opticalfilter within a blood glucose monitor. Optical radiation 115 emittedfrom a light source 110 is focused via a lens assembly 117 (which maycomprise fiber optics or the like) and passes through the filter 120.The dichroic filter 120 comprises an optically transmissive rotatabledisk substrate which is layered with optical coatings having differentthicknesses so as to modulate the broadband optical radiation 115through a spectrum from the near infrared (NIR) (e.g., 700 nm) to theinfrared (IR) (e.g., 1,400 nm) . The filter 120 further includes theoptically opaque strip 122 which may, for example, comprise brass orsome other metal which is deposited radially outward from the center ofthe filter disk 120. The opaque strip provides a "0" location indicatorand zero optical intensity, or electrical offset. The filter disk 120 isdriven in a circular motion by a smooth, disk drive motor in onepreferred embodiment; however, a stepper motor could be usedadvantageously for its known phase condition. Filtered optical radiation125 passes from the filter 120 through a fleshy medium, perfused withblood such as a finger tip 130. In some applications, it may bedesirable to provide a focusing lens, or other optical conduit, betweenthe filter 120 and the finger 130. The light which passes through thefinger 130 is detected by a detector 140. In general, the detectionsignal is conditioned and converted to digital form in the analog todigital conversion circuit 142. The digital signal processor 145 acceptsthe digital signals and accommodates for the imprecision in the dichroicfilter.

In operation, when light 115 is emitted from the broadband light source110 over a wavelength range of approximately 700 nanometers to 1,400nanometers, (or 850-1700 nanometers in another embodiment where theupper and lower wavelengths have a ratio of approximately 2:1) thisbroadband light 115 shines through the rotating dichroic filter 120. Itshould be noted that the light 115 is focused onto a portion of thefilter 120 by means of fiber optics, a lens assembly (e.g., the lens117), or the like. As the dichroic filter 120 rotates, the broadbandlight 115 is filtered through a portion of the dichroic filter 120producing the filtered optical radiation 125. As indicated above, thedichroic filter 120 is coated with optical layers of varying thicknessso that different portions of the dichroic filter 120 pass differentwavelengths of light. Thus, as the filter 120 rotates, the opticalradiation 125 output from the filter includes optical radiation ofvarious wavelengths. In one embodiment, a fiber optic is used to couplethe optical radiation 125 emitted from a portion of the filter 120 tothe patient's finger 120. It should be noted here, that since theoptical characteristics of the filter 120 can be carefully measured andthe rotational speed of the dichroic filter 120 is known, thetime-varying pattern of optical radiation 125 emitted from the filter120 to illuminate the finger 130 is well defined, and therefore, may beused during signal processing to determine the amount of attenuationwhich is due to the optical filter 120.

The optical radiation 125 which is used to illuminate the finger 130passes through the finger 130 to produce the detectable light 135. As iswell known in the art, some of the optical radiation 125 passesunimpeded through the finger 130, some of the optical radiation 125 isreflected within the finger 130 to produce scattering. The scatteredradiation which is transmitted through the finger 130, together with thelight which passes unimpeded through the finger 130, make up the light135. Some of the optical radiation 125 is absorbed by constituentswithin the finger 130.

The finger 130 is known to include a fingernail, skin, bones, flesh, andblood. The blood itself primarily comprises water, oxyhemoglobin,hemoglobin, lipids, protein and glucose. Each of these constituentswithin the finger (e.g., nerves, muscle tissue, etc.) contribute to theabsorption and scattering of the optical radiation 125 through thefinger 130. The absorption of optical radiation through a nonhomogeneousmedium typically follows well defined laws in relation to the opticalcharacteristics of each of the constituents taken separately.Approximations to these laws are expressed in the equations forBeer-Lambert's law, where low scattering applications most closelyfollow the Beer-Lambert equations. The light 135 which passes throughthe finger 130 is incident upon the optical detector 140. The opticaldetector 140 generates an electrical signal proportional to the overallintensity of the light 135.

Although the light 135 typically has different intensities at differentwavelengths, the optical detector 140 generates an electrical signalwhich is proportionate to the area contained under the spectral responsecurve of the light 135 within the optical band detected by the detector140. That is, the optical detector 140 receives light having differentintensities at different wavelengths. The detected wavelengths arerestricted over a band of approximately 850 nm to 1,700 nm due to thecharacteristics of the detector 140, so that, if intensity is plotted asa function of wavelength to obtain a spectral response curve, the areaunder the spectral response curve will be indicative of the averageoptical radiation intensity incident upon the detector 140. Thus, theelectrical signal produced by the detector 140 is proportional to theoverall (i.e., average) intensity of the light 135.

The filter 120 constructed in accordance with the present inventionincludes redundant regions within an entire revolution. Specifically,the filter 120 is symmetrically layered so that the firsthalf-revolution of the filter is substantially symmetrical to the signalof the second half-revolution of the filter 120. That is to say, asdepicted in FIG. 13, the filter is formed in a wedge shape so that thethickness in one direction is constant and the thickness in theperpendicular direction increases linearly. Thus, the secondhalf-revolution of the filter 120 is redundant. For this reason, digitalsamples taken for one-half of the revolution of the filter 120 could bediscarded so that in each rotation of the filter 120 there are 128samples used for purposes of digital signal processing rather than 256samples in one embodiment. Of course, it will be appreciated that someof the samples are lost due to the opaque strip. Alternatively, all 256samples can be used for processing by averaging corresponding values. Inyet an alternative embodiment, the redundant half of the filter may beused for filter and source calibration. Each of the 128 samples (if onlyhalf are used) represents a different portion of the filter 120 havingdifferent optical transmission characteristics.

This application of the optical filter of the present invention isdescribed in a now U.S. patent entitled "BLOOD GLUCOSE MONITORINGSYSTEM," U.S. Pat. No. 5,743,262, filed on the same day as the presentapplication and assigned to the assignee of the present application. Theaforementioned application is hereby incorporated by reference. Theincorporated patent application also provides additional details aboutthe signal processing steps taken to account for the reduced resolutionof the optical filter.

PRODUCTION SPECIFICATIONS FOR THE OPTICAL FILTER

In one advantageous embodiment for blood glucose measurement, theproduction specifications for the filter 120 are as follows:

    ______________________________________                                        SIZE:          20 mm wide × 20 mm wavelength span,                                     linear multilayer coating                                      SUBSTRATE:     25 mm OD glass disc with 7.5 mm shaft                                         hole in center                                                 WAVELENGTH PASSED:                                                                           700-1400 nanometers                                            1/2 BANDWIDTH: 50 to 200 nanometers, bands may repeat                         BLOCKING:      none                                                           ENVIRONMENT:   Survive condensing humidity, 0-70 c                            ______________________________________                                    

The pass band edges are produced so as to differentiate a 20 nanometerband edge.

The pass band may repeat within the window at as little as 400 cm⁻¹spacing, or 17-18 periods within the window. The pass band centertransmission should approach 100%, and the region between pass bandsshould approach 100% reflection.

Blocking requirements outside of the window are not critical. They maybe limited by band-edge materials such as RG660, RG700, orsemiconductors, or O-H bands typically found in glass below 7100 cm⁻¹.

Only the ability to resolve wave number bands near 200 cm⁻¹ with one ormore band edges should limit the cost.

CHARACTERISTICS FOR PRESENT EMBODIMENT

Preferably, the filter will not have a window narrower than 8,000 to11,000 cm⁻¹ or about 910 to 1,250 nm. The bandwidth is advantageouslywider than 200 cm⁻¹, and the band edge is advantageously narrower than200 cm⁻¹. The transmission maximum of the primary band is advantageouslyabove 80%, and the transmission minimum is advantageously below 20%. Anyother bands should be repeatable, unit to unit; but if they are not, acalibration ROM could be used in accordance with the DSP to performinitial calibration of individual filters.

MECHANICAL BOUNDARIES AND CHARACTERISTICS FOR THE PRESENT EMBODIMENT

The linear filter is advantageously rotated about its center at lessthan 4,800 RPM for portable in vivo applications (although near 48,000RPM might be suitable in certain industrial applications), with anaperture centered at a radius of minimum 9 mm to maximum 45 mm, with aclear aperture diameter of 1 mm to 3 mm and a numerical aperture of 0.12to 0.40. The light path passes through a small circular portiontraveling along an annular region of the rotating filter, causing asinusoidal scan of the wavelengths, although they are depositedlinearly.

For dynamic balance and low turbulence, the linear filter is depositedon a circular substrate. Since the center is not used optically, astandard diameter shaft mounting hole is preferred; most of the presenthardware in the invention use either 0.5000-0.000, +0.0005" diameter, or7.5-0.0+0.1 mm. For a small filter, e.g., 20 mm diameter, bonding to theuncoated side would be considered. Note that the filter mount does nothave spokes or other structural interruption of the optical path.

Initial optical-mechanical alignment of the coating on the glass is notcritical beyond 0.5 mm and will be established electronically. Somemarking of the deposit alignment at the edge or center is desired.

Although the preferred embodiment of the present invention has beendescribed and illustrated above, those skilled in the art willappreciate that various changes and modifications to the presentinvention do not depart from the spirit of the invention. Accordingly,the scope of the present invention is limited only by the scope of thefollowing appended claims.

What is claimed is:
 1. An optical filter comprising:a generally circularsubstrate having a top surface and a bottom surface; and a plurality ofoptical coatings deposited on said top surface of said substrate suchthat said coatings vary linearly in thickness in a first directionacross said top surface, said coatings remaining substantially constantin thickness across said top surface in a second direction substantiallyperpendicular to said first direction.
 2. An optical filter comprising:asubstrate having, a top surface and a bottom surface; and a plurality oflayers of optical coatings varying in thickness in a first directionalong, said substrate, said layers providing optical transmissioncharacteristics for said optical filter to provide an optical filterwhich transmits more than one wavelength passband through said filter ateach location across the surface of said filter.
 3. An optical filteraccording to claim 1 further comprising a cylindrical portioncircumferentially disposed about a centered, cylindrical aperture suchthat said filter is rotatably mountable about a shaft disposed throughsaid aperture.
 4. An optical filter according to claim 3 furthercomprising an optically opaque radial strip.
 5. An optical filteraccording to claim 4 wherein there are no more than 17 of said coatings.6. An optical filter according to claim 1 wherein said coatings provideoptical transmission characteristics that are redundant within a regionof an entire revolution of said filter.
 7. An optical filter accordingto claim 6 wherein said transmission characteristics are substantiallysymmetrical between a first half-revolution and a second half-revolutionof said filter.
 8. An optical filter according to claim 7 wherein aparticular wavelength has a plurality of transmission passbands oversaid first half-revolution.
 9. An optical filter according to claim 7wherein there are transmission passbands for a plurality of wavelengthsat each rotational position of said filter.
 10. An optical filteraccording to claim 7 wherein approximately one-half of the incidentlight is transmitted through said coatings at each rotational positionof said filter.
 11. An optical filter according to claim 7 wherein aratio of overall transmitted signal intensity to transmitted signalintensity for any particular wavelength is in the range of 10 to 100 ateach rotational position of said filter.
 12. An optical filter accordingto claim 2 wherein approximately one-half of the incident light istransmitted through said layers at each location across the surface ofsaid filter.
 13. An optical filter according to claim 2 wherein a ratioof overall transmitted signal intensity to transmitted signal intensityfor any particular wavelength is in the range of 10 to 100 at eachlocation across the surface of said filter.
 14. An optical filtercomprising:a generally circular substrate having a surface and aplurality of rotational positions around said surface; and a pluralityof optical coatings deposited on said surface which vary linearly inthickness in a first direction across said surface and remainsubstantially constant in thickness in a second direction substantiallyperpendicular to said first direction, providing transmissioncharacteristics as a function of wavelength and said rotationalpositions.
 15. An optical filter according to claim 14 wherein saidtransmission characteristics include a plurality of passbands for aparticular wavelength.
 16. An optical filter according to claim 15wherein said transmission characteristics include a plurality ofpassbands for each of a plurality of wavelengths.
 17. An optical filtercomprising:a generallv circular substrate having a surface and aplurality of rotational positions around said surface; and a pluralityof optical coatings deposited on said surface which vary in thickness ina first direction and remain substantially constant in thickness in asecond direction substantially perpendicular to said first direction,providing transmission characteristics as a function of wavelength andsaid rotational positions, said transmission characteristics including aplurality of passbands for each of a plurality of wavelengths. whereinsaid wavelengths are in a range from 700 to 1,400 nanometers.
 18. Anoptical filter according to claim 17 wherein said wavelengths are in arange no narrower than 910 to 1,250 nanometers
 19. An optical filteraccording to claim 14 wherein said transmission characteristics includemultiple passbands at each of said rotational positions.
 20. An opticalfilter according to claim 19 wherein approximately one-half of theincident light is transmitted through said coatings at each of saidrotational positions.
 21. An optical filter according to claim 19wherein a ratio of overall transmitted signal intensity to transmittedsignal intensity for any particular wavelength is in the range of 10 to100 each of said rotational positions.